1. Field of Invention
The present invention relates to an apparatus and a method for measuring quality of an optical signal, which are used in a monitoring system of an optical network to detect the quality factor (Q-factor) of the optical signal and to evaluate the bit error rate (BER) of the optical signal.
2. Related Art
With the development of high speed optical networks and all-optical networks, transmission rates have increased to tera-bit per second and transmission distances have become longer and longer. When an optical signal is transmitted in a channel, the quality of the data transmitted will be influenced by many factors. Thus, a desirable optical signal quality monitoring system must be established to effectively manage the optical network and improve the efficiency thereof. However, with the gradual decrease in the number of conventional electrical nodes, the electric network performance monitors with the conventional electrical nodes have gradually disappeared. Therefore, a method suitable for an all-optical network and with functionality equivalent to that of an electric network performance monitor must be developed.
Referring to FIG. 1, it is an optical network 10. An optical signal is transmitted from an optical node 101 disposed in the optical network 10 to another optical node 102 through the wavelength division multiplexing (WDM) technology 110. Optical performance monitoring (OPM) is performed in the optical channel to detect the quality of the optical channel. Through the optical performance monitoring, the power, the wavelength, the signal-to-noise ratio, and the like of the optical signal can be measured. However, in order to more accurately monitor the quality of the signal, especially for a digital transmission system, there are generally two parameters for evaluating the performance: BER and Q-factor. The detection of the BER requires a relatively complicated setting, and the lower the BER is, the longer the measuring time will be. For example, for a 10 Gbps signal, when the detected BER is reduced to 10E-12, i.e., one bit error appears for every 1012 bits transmitted, the measurement time required is 100 seconds. When the detected BER is reduced to 10E-13, the measurement time required is 1000 seconds. When the detected BER is reduced to 10E-15, the measurement time required is 27 hours. Therefore, through this method, some real-time audio-visual signals cannot be monitored in real time, thus errors cannot be found out instantly and effectively. In this case, since the Q-factor can be used to evaluate the BER in principle and can be rapidly measured, the Q-factor, instead of the BER, can serve as the parameter to evaluate system performance. A higher Q-factor corresponds to a lower BER and indicates an optical signal with a higher quality. Additionally, the BER cannot be measured until all signals have been received, such that it can only be measured at a receiving end. The Q-factor can be measured at any time during the transmission of the signal without interrupting the transmitted signal. Therefore, compared with the measurement of the BER, the measurement of the Q-factor is more flexible and more suitable for real-time optical channel transmission systems.
The definition of the Q-factor is represented by Equation (1):
                    Q        =                                                                                            μ                  1                                -                                  μ                  0                                                                                                  σ                1                            +                              σ                0                                              .                                    (        1        )            
In equation (1), μ1 and μ0 respectively represent the average value of individual measurements when the received optical signal is at level “1” and level “0”; while σ1 and σ0 respectively represent the standard deviation of individual measurements when the received optical signal is at level “1” and level “0”. If the noise probability distribution at the receiving end is a Gaussian distribution, and the intersymbol interference (ISI) can be neglected, the relationship expression between the Q-factor and the BER can be represented by Equation (2):
                    BER        =                                            1              2                        ⁢                          erfc              ⁡                              [                                  Q                                      2                                                  ]                                              ≈                                                    exp                (                                                      -                                          Q                      2                                                        2                                )                                            Q                ⁢                                                      2                    ⁢                    π                                                                        .                                              (        2        )            
Referring to FIG. 2, it is a curve 20 of the Q-factor corresponding to the BER. The transverse axis in the curve represents the Q-factor 201 and the longitudinal axis represents the BER 202. The corresponding relationship between the Q-factor and the BER forms a curve. It can be seen from FIG. 2 that, the BER becomes smaller as the Q-factor becomes larger.
A Q-factor measuring module can be implemented in an optical network transmission system architecture. Referring to FIG. 3, it shows a Q-factor parameter monitoring system architecture 30. A tunable laser diode 301 generates an optical signal 931, which is then modulated by an electro-optic modulator 302. Then, the optical signal 931 is further transmitted to an erbium doped fiber amplifier (EDFA) 304 via an optical fiber 303. The erbium doped fiber amplifier (EDFA) amplifies the power of the optical signal, and inputs the optical signal 931 into an optical splitter 305. Then, the optical splitter 305 splits the optical signal 931 into two parts, wherein one part is transmitted to a next optical node 306; and the other part is input into a Q-factor measuring module 307. Thus, the Q-factor measuring module may finish measuring the Q-factor in cooperation with a signal processor 308 without interrupting the transmission of a network signal. Therefore, the operation of the network will not be influenced.
However, in order not to influence the normal operation of the network, the power of the optical signal assigned to be monitored by the Q-factor measuring module 307 is generally quite small, which requires the monitoring module with high measurement sensitivity, such that the complexity and cost of equipment are increased. In conventional optical communication, a coherent detection technique can be used to detect a weak received signal, which will not generate a great amount of noise or change the features of the signal. The coherent detection requires a local oscillator to generate a continuous wave with the same wavelength/frequency and phase, which is also called homodyne. Furthermore, the power of continuous wave is many times larger than that of the weak received signal, such that the weak received signal can be amplified without generating additional noise and influencing the original features. Referring to FIG. 4, it is a coherent detection module 40 of the prior art. The tunable laser diode 401 is a local oscillator that generates an optical signal LO(t). A wideband optical frequency locking loop 405 is used to lock the optical signal LO(t) of the tunable laser diode 401 at the wavelength that is the same as that of a tested optical signal 941, wherein the tested optical signal 941 is just one part of the optical signal 931 that enters into the Q-factor measuring module 307 after being split by the optical splitter 305. The base equation is listed as follows:S(t)=m(t)cos(ωct)  (3);LO(t)=A cos(ωct)  (4);{S(t)+LO(t)}2=S2(t)+LO2(t)+2S(t)×LO(t)  (5);S(t)×LO(t)=½[Am(t)+Am(t)cos(2ωct)]  (6).
In Equation (3), S(t) represents the tested optical signal 941; m(t) represents a baseband signal; and wc represents a frequency of an optical carrier wave. In Equation (4), LO(t) represents the signal generated by the tunable laser diode 401. Through the wideband optical frequency locking loop 405, LO(t) generated by the tunable laser diode 401 may have the same wavelength as that of S(t). A is an amplitude of LO(t) and represents a power strength that is often much larger than that of m(t). The optical signals S(t) and LO(t) with the same wavelength enter into an optical receiver 403 after being coupled by the optical coupler 402. As expressed by Equations (5) and (6), high-frequency signals S2(t), LO2(t), and Am(t)cos(2ωct) in the coupled signal {S(t)+LO(t)}2 are filtered out by a filter in the optical receiver 403. Finally, an amplified output signal 942, i.e., Am(t), is obtained from the signal processor 404, wherein the output signal 942 is A times larger than the baseband signal m(t). It can be known that the coherent detection is used to detect the weak signal, so it amplifies the weak signal and reconstruct the waveform.
As for a conventional coherent detection module, such as that disclosed in U.S. Pat. No. 7,042,629 issued on May 9, 2006, in the Q-factor monitoring technology, a wavelength tunable optical pulse laser with the same wavelength as that of a tested optical signal and the tested optical signal are coupled together to enter into the optical receiver. An optical pulse generated by the wavelength tunable optical pulse laser is used to replace the aforementioned continuous wave to amplify and sample the signal, wherein the process of providing the same wavelength is achieved through a wideband optical frequency locking loop. However, although such architecture achieves Q-factor monitoring, the complexity and the equipment cost are relatively high, and it is difficult for the wideband optical frequency locking loop to achieve the phase coherence of the optical signal during high frequency operation.